COVID-19 disease, caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV- 2) infection, has become a major threat to global public health [
28,
29]. In many patients, the symptoms remain mild (only upper respiratory tract infections with mild fever and without shortness of breath or hypoxia, SpO2 > 95%) and resolve automatically within four to seven days by symptom-based medication. Some patients who developed primary ARDS-like conditions need hospitalization with moderate severity (respiratory rate ≥ 24/min, breathlessness, and SpO2 level 90% to ≤ 93% on room air). A maximum number of patients showed moderate disease-like symptoms between 8 and 15 days after the first symptoms. However, 2–5% of COVID-19 patients developed severe ARDS-like conditions with frequent breathing problems, low SpO2 (< 90%), and high CT scores (> 16) [
30,
31]. A higher number of patients were reported after the 17th day of the disease. Syncytia formation is associated with the severity of COVID-19. However, it is possible that syncytia formation may also be related to the duration of the infection, as well as other factors such as the patient’s immune response and viral load. It is still not fully understood how syncytia formation contributes to the pathogenesis of COVID-19. Thus, we have taken both parameters as disease severity and the day of sample collection after the infection to equalize the patient conditions.
Histological imaging of lung autopsy samples (death due to COVID-19) showed multinucleate giant syncytial cells [
12,
15,
16,
20]. However, it is difficult to explore the initiation of syncytia formation and its correlation with the disease stage from these autopsy samples. This is because light microscopy-based histological techniques are unsuitable for identifying the respiratory and hemopoietic cells involved in syncytia formation. Our electron microscopy-based ultrastructural study of BALF samples from mildly infected COVID-19 patients did not find syncytia (Additional file 1. Figure
S1). This indicates that SARS-CoV-2 viruses might not initiate membrane fusion at the initial stage of the disease. This may be because the virus tends to multiply, tropism, and infects the maximum number of cells to propagate, including the oral and respiratory epithelium, in the initial phase of the disease [
32,
33]. At this stage, the virus does not need special facilitation for its replication, dissemination, immune evasion, and inflammatory responses because the host immunological response was not yet activated efficiently. However, in the moderate stage of the disease, a negative pressure was created on the replication and dissemination of the SARS-CoV-2 virus by activating humoral and cellular immunological responses [
34]. Macrophage activation and granulocyte infiltration in lung tissue (neutropenia) force the virus to activate its safeguard mechanism to facilitate its replication and dissemination through the fusion of the plasma membrane of respiratory and hemopoietic cells [
20,
35,
36]. This could be a reason for plasma membrane fusion with identical and heterotypic cells at the moderate stage of the disease (Figs.
1,
2 and
3). Entirely fused binucleate type II pneumocyte syncytial cells were observed in this disease stage (Fig.
1). However, in a similar stage of the disease, incomplete plasma membrane fusion (initiation of membrane fusion) was observed in hemopoietic origin cells such as monocytes and neutrophils (Figs.
2 and
3). This indicates that type II pneumocytes were the first choice of the virus to induce syncytial cell formation, which may help in the replication and protection of the virus from the immune response [
21]. This could be the reason for high fluorescence (indicative of high infection and multiplication) in type II pneumocyte-origin syncytial cells (Fig.
1). A recent study also supported this, which provided critical insights and the consequences of multinucleated giant cell formation after SARS-CoV-2 infections [
16,
20]. Further, the formation of homologous syncytia was also reported
in vitro Vero cells (ACE2+) expressing SARS-CoV-2 spike protein [
21].
It is hypothesized that syncytia may also contribute to viral dissemination and immune evasion by protecting the virus from immune cells and from neutralizing antibodies [
1,
12]. In our study, the initiation of identical (in neutrophils) and heterotypic cell fusion (with neutrophils and monocytes) in the moderate stage of the disease indicates the hyperactivation of viral fusogen proteins to initiate the syncytia formation in these hematopoietic cells (Figs.
2 and
3). It was reported that the viral spike (S) protein worked as a fusogen on the surface of an infected cell which interacts with receptors on neighboring cells to form syncytial cells [
37]. Besides its ability to drive the fusion of viral and cellular membranes, the S protein can further drive the fusion of neighboring cells, which results in the formation of multinucleated giant cells [
12,
14,
15]. This finding was also supported by a recent study in which SARS-CoV-2-induced syncytia targeted the lymphocytes for internalization and cell-in-cell mediated elimination, potentially contributing to lymphopenia and pathogenesis in COVID-19 patients [
20].
The finding of mature and very large size syncytial cells (20–100 μm) of neutrophils and monocytes origin at very late and severe stages (ARDS-like) of the diseases indicated the role of syncytial cells in the viral dissemination and immune evasion. These giant syncytial cells may help in the multiplication and protection from the body’s immune response, which may be the reason for the alveolar macrophage activation and, ultimately, the cytokine storm [
20]. Mature syncytial cells were not identified in our TEM imaging though it was reported in the autopsy samples. This could be attributed to the release of fewer such kinds of cells from the infected lung tissue to the BALF of the patients.